Conventional edge emitting semiconductor lasers are playing a significant role in development of optical communications due to their high operating efficiency, small size and modulation capabilities. However, with increasing performance needs, further reduction in size, and greater manufacturing needs, these devices have been experiencing limitations.
Recently, there has been an increased interest in a new type of laser device called a vertical cavity surface emitting laser (VCSEL). Advantages of VCSEL devices are that the device is smaller, has potentially higher performance, and is potentially more manufacturable. These advantages are due in part from advances in epitaxial deposition techniques such as metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE).
However, even with these advances in deposition techniques there is difficulty during manufacturing in controlling the mode of operation of the laser and in controlling current distribution within the laser. In general, VCSELs are formed by depositing a plurality of layers on a substrate and then etching the layers down to the substrate to form the VCSEL. See, for example, U.S. Pat. No. 5,034,092, entitled "Plasma Etching of Semiconductor Substrates" issued Jul. 23, 1991, assigned to the same assignee and included herein by reference.
The etching of mesas to form VCSELs has two disadvantages. The etching process damages the crystal at the surface and leads to increased threshold currents and reduced reliability. The mesas form a waveguide with a large discontinuity in the refractive index which makes it very difficult to control the optical modes without making devices of very small dimension, which increases the series resistance and reduces the maximum output power. Generally this results in devices which are less efficient and less stable. Other problems include thermal changes during operation which alter thicknesses and current flow to produce changes known as self-focusing effects. These self-focusing effects, along with others, either reduce the optical mode size or cause higher order mode operation.
Another problem in VCSELs is the series resistance between electrical contacts. As the number of layers in the mirror stacks increases, the series resistance increases which reduces the efficiency of the VCSEL.
Accordingly, it is a purpose of the present invention to provide a new and improved vertical cavity laser.
Another purpose of the present invention is to provide a new and improved vertical cavity laser with a larger mode size.
It is still another purpose of the present invention to provide a new and improved vertical cavity laser with reduced series resistance.
It is a further purpose of the present invention to provide a new and improved method of fabricating vertical cavity lasers.